The stress-strain curve is a useful tool for understanding the behavior of steel bars under loads. This curve is created by subjecting steel specimens to gradual pulling until they break. The stress and corresponding strains are recorded during this process, and then plotted on a graph. The stresses are plotted on the vertical axis, while the corresponding strains are plotted on the horizontal axis.
The stress-strain curve has different mark points that represent the various stages the steel specimen passes through prior to fracture. Understanding this curve is crucial to understanding how steel bars respond to loads.
Initially, the steel specimen behaves as an elastic material, where stresses and strains are proportional. However, as the load increases, the specimen starts to lose its proportionality and eventually fails or yields. Once the load surpasses the yield point, the steel bar goes through stress hardening, which allows it to sustain greater stress. Eventually, the steel bar will reach its fracture point and fail.
Stress-strain Curve of Steel Bars
The given context describes the different stages that a steel specimen goes through when it is subjected to a load. These stages are represented on a stress-strain diagram. The diagram is used to visualize the relationship between the applied force or stress and the resulting deformation or strain of the material.
The first stage that a steel specimen goes through is the elastic stage. During this stage, the material is able to deform elastically, meaning that it can return to its original shape once the load is removed. This behavior is described as linear on the stress-strain diagram and is characterized by the proportionality of stress to strain.
The second stage is the yield point. At this point, the material has reached its maximum stress, and further deformation will result in permanent plastic deformation. The yield point is the stress at which this plastic deformation begins. On the stress-strain diagram, this point is represented by a sudden drop in stress with increasing strain.
The final stage is fracture. If the applied load is too great, the steel specimen will fracture or break. This point is characterized by a steep drop in stress with little or no increase in strain on the stress-strain diagram.
Understanding the different stages of the stress-strain diagram is important for materials scientists and engineers, as it can inform the design of materials and structures. By knowing the yield point and fracture point of a material, for example, engineers can design structures that will not fail under normal operating conditions.
Limit of Proportionality
The stress-strain curve represents the behavior of a material when subjected to stress. The initial stage of this curve extends from the starting point to point “A”. During this stage, the stress is relatively low and does not cause any permanent deformation to the material. In other words, the material returns to its original shape once the stress is removed. Additionally, the stress and strain values are directly proportional to each other in this stage. Hence, any increase or decrease in stress causes an equivalent change in the strain of the material. Therefore, the behavior of the material during this stage is elastic in nature.
Elastic Limit
Between points “A” and “B” on the curve, the steel specimen is subjected to a certain level of stress. If this stress is increased further, the specimen would undergo elastic strain, which means that it would deform in response to the applied stress, but would return to its original shape once the stress is removed. However, the relationship between the stress and strain in this region is not proportional to each other. This means that the amount of strain generated by a given amount of stress is not constant, and instead varies depending on the specific characteristics of the material being tested. Overall, this region of the stress-strain curve represents an important phase in the behavior of steel and other materials, as it can help engineers and scientists understand how they will respond to different levels of stress and deformation.
Yield Point
The stress-strain curve has a crucial point from a design perspective known as point B. It is deemed to be the failure point in the design of reinforced concrete structures and is denoted by the letter B on the curve. As the steel bar in the reinforced concrete element approaches the yield point, it would be considered a failed member. The yield point is the point where steel starts to experience plastic deformation, and the stress and strain are no longer proportional. Point B on the curve is referred to as the upper yield point, whereas point C is known as the lower yield point.
Ultimate Strength
When a material is subjected to stress beyond its yield point, it undergoes a phenomenon known as strain hardening. This is represented by the segment of the stress-strain curve between point C and D. During this process, the material undergoes changes in its atomic and crystalline structure, resulting in an increased resistance to further deformation. The ultimate strength or tensile strength of the material is represented by the maximum point on the stress-strain curve, which is point D. Beyond this point, the material begins to undergo necking, which is the localized thinning of the material under stress. In other words, strain hardening is a process that increases the material’s strength, but it is not a sustainable process and eventually leads to failure.
Rupture Strength
The strength of a material at the point of rupture, also referred to as breaking strength, is known as rupture strength. This measurement is represented by point “E” on the stress-strain diagram.